A variety of physical conditions involve two tissue surfaces that, for treatment of the condition, need to be distracted from one another and then supported away from one another. Such distraction may be to gain exposure to select tissue structures, to apply a therapeutic pressure to select tissues, to return tissue structures to their anatomic position and form, or in some cases to deliver a drug or growth factor to alter, influence or deter further growth of select tissues. Depending on the condition being treated, the tissue surfaces may be opposed or contiguous and may be bone, skin, soft tissue, or a combination thereof. An optimal treatment method includes distracting and supporting the tissue surfaces simultaneously.
A minimally invasive distraction and support device would have significant application in orthopaedic surgical procedures, including acute and elective procedures to treat bone fractures and degenerative changes of the skeletal system and including vertebral compression fractures, interbody fusion, vertebral disc augmentation or replacement, and other compression fractures including, but not limited to tibial plateau compression fractures, calcaneous compression fractures, distal tibia fractures, distal radius (wrist) fractures, crushed or fractured orbit and orthopaedic oncology. Further, a minimally invasive distraction and support device would have application in non-orthopaedic surgical procedures in plastic surgery (for example facial reconstruction), gastrointestinal surgery and urological surgery (for example the treatment of incontinence).
Vertebral Compression Fractures
A vertebral compression fracture is a crushing injury to one or more vertebrae. Vertebral fractures are generally associated with osteoporosis (the “brittle bone” disease), metastasis, and/or trauma. Osteoporosis reduces bone density, thereby weakening bones and predisposing them to fracture.
The osteoporosis-weakened bones can collapse during normal activity. In severe cases of osteoporosis, actions as simple as bending forward can be enough to cause a vertebral compression fracture. Vertebral compression fractures are the most common type of osteoporotic fractures according to the National Institute of Health. The mechanism of these fractures is one of flexion with axial compression where even minor events cause damage to the weak bone. While the fractures may heal without intervention, the crushed bone may fail to heal adequately. Moreover, if the bones are allowed to heal on their own, the spine will be deformed to the extent the vertebrae were compressed by the fracture. Spinal deformity may lead to breathing and gastrointestinal complications, and adverse loading of adjacent vertebrae.
Vertebral fractures happen most frequently at the thoracolumbar junction, with a relatively normal distribution of fractures around this point. Vertebral fractures can permanently alter the shape and strength of the spine. Commonly, they cause loss of height and a humped back. This disorder (called kyphosis or “dowager's hump”) is an exaggeration of the spinal curve that causes the shoulders to slump forward and the top of the back to look enlarged and humped. In severe cases, the body's center of mass is moved further away from the spine resulting in increased bending moment on the spine and increased loading of individual vertebrae.
Another contributing factor to vertebral fractures is metastatic disease. When cancer cells spread to the spine, the cancer may cause destruction of part of the vertebra, weakening and predisposing the bone to fracture.
Osteoporosis and metastatic disease are common root causes leading to vertebral fractures, but trauma to healthy vertebrae also causes minor to severe fractures. Such trauma may result from a fall, a forceful jump, a car accident, or any event that stresses the spine past its breaking point. The resulting fractures typically are compression fractures or burst fractures.
Vertebral fractures can occur without pain. However, they often cause a severe “band-like” pain that radiates from the spine around both sides of the body. It is commonly believed that the source of acute pain in compression fractures is the result of instability at the fracture site, allowing motion that irritates nerves in and around the vertebrae.
Until recently, treatment of vertebral compression fractures has consisted of conservative measures including rest, analgesics, dietary, and medical regimens to restore bone density or prevent further bone loss, avoidance of injury, and bracing. Unfortunately, the typical patient is an elderly person who generally does not tolerate extended bed rest well. As a result, minimally invasive surgical methods for treating vertebral compression fractures have recently been introduced and are gaining popularity.
One technique used to treat vertebral compression fractures is injection of bone filler into the fractured vertebral body. This procedure is commonly referred to as percutaneous vertebroplasty. Vertebroplasty involves injecting bone filler (for example, bone cement) into the collapsed vertebra to stabilize and strengthen the crushed bone.
In vertebroplasty, physicians typically use one of two surgical approaches to access thoracic and lumbar vertebral bodies: transpedicular or extrapedicular. The transpedicular approach involves the placement of a needle or wire through the pedicle into the vertebral body, and the physician may choose to use either a unilateral access or bilateral transpedicular approach. The second approach, the extra-pedicular technique, involves an entry point through the posterolateral corner of the vertebral body. The needle entry point is typically 8 cm to 12 cm lateral of the mid-sagittal plane, with the skin incision typically closer to 8 cm in the proximal spine and generally closer to 12 cm in the distal spine. In general, one cannula is placed to fill the vertebral body with the extra-pedicular approach.
Regardless of the surgical approach, the physician generally places a small diameter guide wire or needle along the path intended for the bone filler delivery needle. The guide wire is advanced into the vertebral body under fluoroscopic guidance to the delivery point within the vertebrae. The access channel into the vertebra may be enlarged to accommodate the delivery tube. In some cases, the delivery tube is placed directly and forms its own opening. In other cases, an access cannula is placed over the guide wire and advanced into the vertebral body. After placement, the cannula is replaced with the delivery tube, which is passed over the guide pin. In both cases, a hollow needle or similar tube is placed into the vertebral body and used to deliver the bone filler into the vertebra.
In this procedure, lower viscosities and higher pressures tend to disperse the bone filler throughout the vertebral body. However, such conditions dramatically increase the risk of bone filler extravasation from the vertebral body. The transpedicular approach requires use of a relatively small needle (generally 11 gauge or smaller). In contrast, the extrapedicular approach provides sufficient room to accommodate a larger needle (up to 6 mm internal diameter in the lumbar region and lower thoracic regions). In general, the small diameter needle required for a transpedicular approach necessitates injecting the bone filler in a more liquid (less viscous) state. Further, the pressure required to flow bone filler through a small gauge needle is relatively high. The difficulty of controlling or stopping bone filler flow into injury sensitive areas increases as the required pressure increases. The larger needle used in the extrapedicular approach allows injection of bone filler in a thicker, more controllable viscous state. Therefore, many physicians now advocate the extrapedicular approach so that the bone filler may be delivered through a larger cannula under lower pressure.
Caution must be taken to prevent extravasation, with the greatest attention given to preventing posterior extravasation because it may cause spinal cord trauma. Physicians typically use fluoroscopic imaging to monitor bone filler propagation and to avoid flow into areas of critical concern. If a foraminal leak results, the patient may require surgical decompression and/or suffer paralysis.
Kyphoplasty is a modified vertebral fracture treatment that uses one or two balloons, similar to angioplasty balloons, to attempt to reduce the fracture and restore vertebral height prior to injecting the bone filler. Two balloons are typically introduced into the vertebra via bilateral transpedicular cannulae. The balloons are inflated to reduce the fracture. After the balloon(s) is deflated and removed, leaving a relatively empty cavity, bone cement is injected into the vertebra. In theory, inflation of the balloons restores vertebral height. However, it is difficult to consistently attain meaningful height restoration. It appears the inconsistent results are due, in part, to the manner in which the balloon expands in a compressible media and the structural orientation of the trabecular bone within the vertebra.
Tibial Plateau Compression Fractures
A tibial plateau fracture is a crushing injury to one or both of the tibial condyles resulting in a depression in the articular surface of the condyle. In conjunction with the compression fracture, there may be a splitting fracture of the tibial plateau. Appropriate treatment for compression fractures depends on the severity of the fracture. Minimally displaced compression fractures may be stabilized in a cast or brace without surgical intervention. More severely displaced compression with or without displacement fractures are treated via open reduction and internal fixation.
Typically, the underside of the compression fracture is accessed either through a window cut (a relatively small resection) into the side of the tibia or by opening or displacing a splitting fracture. A bone elevator is then used to reduce the fracture and align the articular surface of the tibial condyle. A fluoroscope or arthroscope may be used to visualize and confirm the reduction. Bone filler is placed into the cavity under the reduced compression fracture to maintain the reduction. If a window was cut into the side of the tibia, the window is packed with graft material and may be secured with a bone plate. If a splitting fracture was opened to gain access, then the fracture is reduced and may be stabilized with bone screws, bone plate and screws, or a buttress plate and screws. (Both of these methods are very invasive and require extensive rehabilitation.)
Spinal Interbody Fusion
Spinal fusion is most frequently indicated to treat chronic back pain associated with instability or degenerative disc disease that has not responded to less invasive treatments. Fusion is also prescribed to treat trauma and congenital deformities. Spinal fusion involves removal of the spinal disc and fusing or joining the two adjacent vertebrae. The primary objective for patients suffering from instability is to diminish the patient's pain by reducing spinal motion.
Spinal fusions are generally categorized into two large groups: instrumented and non-instrumented. In non-instrumented procedures, the physician removes tissue from the unstable disc space and fills it with some form of bone graft that facilitates the fusion of the two adjacent vertebral bodies. Instrumented procedures are similar to non-instrumented procedures, except that implants (generally metallic) are also applied to further stabilize the vertebrae and improve the likelihood of fusion.
Conventional instrumented procedures generally utilize plates, rods, hooks, and/or pedicle screws through various surgical approaches. These conventional implants are secured to the vertebral bodies that are being fused. Interbody fusion devices were introduced in the 1990's as a less invasive surgical alternative, although interbody devices are increasingly being used in conjunction with pedicle screws. Interbody devices are implanted into the disc space to restore the normal disc spacing, utilizing tension in the annulus to stabilize the fusion unit. Interbody fusion provides a large area of the vertebral end plate for establishing bony fusion, a viable blood supply from the decorticated end plates, and dynamic compressive loading of the graft site. The interbody devices are generally filled with a bone filler to facilitate fusion. Interbody devices can be categorized in three primary groups: spinal fusion cages, which are available in a variety of shapes including rectangular, round-faced, and lordotic; allograft bone dowels and wedges (which are also available in various shapes); and titanium mesh (although titanium mesh is not itself a structural spacer).
Interbody fusion is typically completed through a posterior, an anterior, or a lateral intertransverse approach. Each of these techniques has limitations. Lumbar interbody fusion presents a challenging surgical procedure and relatively high pseudoarthrosis rates. As a result, this approach is increasingly combined with additional internal fixation devices such as pedicle screw fixation.
In all interbody surgical approaches, a relatively large opening is made in the annulus. The nuclear material is removed and the end plates are decorticated to facilitate bony fusion. Overall, the use of interbody devices has resulted in mixed clinical outcomes. Placement of a fixed height device presents challenges in proper tensioning of the annulus. For these and other reasons, there is concern over long-term stability of interbody devices and fusion mass.